Genetic differentiation of Verticillium dahliae populations recovered from symptomatic and asymptomatic hosts.

Verticillium dahliae is a soilborne fungal pathogen affecting many economically-important crops that can also infect weeds and rotational crops with no apparent disease symptoms. The main goal was to test the hypothesis that V. dahliae populations recovered from asymptomatic rotational crops and weed species are genetically distinct from those recovered from symptomatic hosts. We collected V. dahliae isolates from symptomatic and asymptomatic hosts growing in fields with histories of Verticillium wilt of potato in Israel and Pennsylvania (PA, US), and used genotyping-by-sequencing (GBS) to analyze the evolutionary history and genetic differentiation between populations of different hosts. A phylogeny inferred from 26,934 SNPs in 126 V. dahliae isolates displayed a highly clonal structure correlated with vegetative compatibility groups, and isolates grouped in lineages 2A, 2B824, 4A and 4B, with 77% of the isolates in lineage 4B. The lineages identified in this study were differentiated by host of origin; we found 2A, 2B, and 4A only in symptomatic hosts, but isolates from asymptomatic hosts (weeds, oat, and sorghum) grouped exclusively within lineage 4B, and were genetically indistinguishable from 4B isolates sampled from symptomatic hosts (potato, eggplant, and avocado). Using coalescent analysis of 158 SNPs of lineage 4B, we inferred a genealogy with clades that correlated with geographic origin. In contrast, isolates from asymptomatic and symptomatic hosts shared some of the same haplotypes and were not differentiated. We conclude that asymptomatic weeds and rotational hosts may be potential reservoirs for V. dahliae populations of lineage 4B, which are pathogenic to many cultivated hosts.